Although a great deal of progress has been made in controlling acute rejection in solid organ allografts, chronic rejection and progressive graft loss over the years remains a significant problem. The potential for inducing specific immunological tolerance has been a long time goal which continues to be a promising strategy for avoiding indefinite immunosuppression and chronic rejection. Persistent donor lymphohematopoietic chimerism has recently been recognized in patients with long term stable functioning grafts suggesting that the presence of certain lymphoid or progenitor cells may facilitate tolerance. We propose to study patients receiving transplants from HLA haploidentical living related donors (LRD) who prior to transplantation receive a modified donor specific transfusions (DST). The cells infused during DST will be varied to determine the effects of mature lymphoid cells and CD34+ hematopoietic progenitor cells. The end points examined include changes in donor specific immune responses, and the frequency and extent of donor cell chimerism occurring after DST and following transplantation. Our Specific Aims are to: 1) characterize the types of donor cells occurring in chimeric kidney transplant patients and their hematopoietic stem cell activity; 2) analyze the immunological changes occurring in chimeric patients; 3) determine the conditions for achieving optimal sustained hematopoietic chimerism; and 4) determine if hematopoietic chimerism can facilitate the induction of specific tolerance and improve kidney allograft survival. In order to optimize the effectiveness of DST, we will perform a series of phase I/Il clinical trials to determine which type of donor cells lead to enduring donor chimerism and cause the most profound changes in donor specific immune responses as measured in vitro by mixed lymphocyte culture (MLC) assays and analysis of cytotoxic T lymphocyte precursor (CTLp) frequencies by limiting dilution analysis (LDA). Prior sensitization to alloantigen by blood transfusion or pregnancy may represent a barrier to establishing donor chimerism, but it is possible that this problem can be circumvented by infusion of donor CD34+ progenitor cells depleted of antigen presenting cells (APC). if depletion of APC alone fails to overcome the risk of alloimmunization, we will test the alternative hypothesis that prior allosensitization can be reversed and donor chimerism established if CD34+ progenitor cells are given together with immunosuppression. The chimerism and immunological studies to be performed at different time points after DST and following transplantation should elucidate the relationship between chimerism and tolerance, and help define the mechanisms responsible for the specific nonresponsiveness observed in patients with long term stable graft function.